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<title>FFTW 3.3.8: Extended and quadruple precision in Fortran</title>

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<a name="Extended-and-quadruple-precision-in-Fortran"></a>
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Previous: <a href="Overview-of-Fortran-interface.html#Overview-of-Fortran-interface" accesskey="p" rel="prev">Overview of Fortran interface</a>, Up: <a href="Overview-of-Fortran-interface.html#Overview-of-Fortran-interface" accesskey="u" rel="up">Overview of Fortran interface</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Extended-and-quadruple-precision-in-Fortran-1"></a>
<h4 class="subsection">7.1.1 Extended and quadruple precision in Fortran</h4>
<a name="index-precision-7"></a>

<p>If FFTW is compiled in <code>long double</code> (extended) precision
(see <a href="Installation-and-Customization.html#Installation-and-Customization">Installation and Customization</a>), you may be able to call the
resulting <code>fftwl_</code> routines (see <a href="Precision.html#Precision">Precision</a>) from Fortran if
your compiler supports the <code>C_LONG_DOUBLE_COMPLEX</code> type code.
</p>
<p>Because some Fortran compilers do not support
<code>C_LONG_DOUBLE_COMPLEX</code>, the <code>fftwl_</code> declarations are
segregated into a separate interface file <code>fftw3l.f03</code>, which you
should include <em>in addition</em> to <code>fftw3.f03</code> (which declares
precision-independent &lsquo;<samp>FFTW_</samp>&rsquo; constants):
</p>
<a name="index-iso_005fc_005fbinding-2"></a>
<div class="example">
<pre class="example">  use, intrinsic :: iso_c_binding 
  include 'fftw3.f03'
  include 'fftw3l.f03'
</pre></div>

<p>We also support using the nonstandard <code>__float128</code>
quadruple-precision type provided by recent versions of <code>gcc</code> on
32- and 64-bit x86 hardware (see <a href="Installation-and-Customization.html#Installation-and-Customization">Installation and Customization</a>),
using the corresponding <code>real(16)</code> and <code>complex(16)</code> types
supported by <code>gfortran</code>.  The quadruple-precision &lsquo;<samp>fftwq_</samp>&rsquo;
functions (see <a href="Precision.html#Precision">Precision</a>) are declared in a <code>fftw3q.f03</code>
interface file, which should be included in addition to
<code>fftw3l.f03</code>, as above.  You should also link with
<code>-lfftw3q -lquadmath -lm</code> as in C.
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